overloaded cf_promises_validated or validated_<input> with -f <input> timestamp use

Agent use of cf_promises_validated as parsed time causes conflicts for workdirs with any other component running which can update the global validated mechanism. My canonical example is cf-execd running with cf-serverd, very likely with policy server.

1. With both cf-execd and cf-serverd running with default input of 'promises.cf'
2. touch/modify promises.cf in inputs directory
3. (Very likely) cf-serverd will see this change, rerun cf-promises and update workdir cf_promises_validated
4. cf-execd calls NewPromiseProposals, which interprets cf_promises_validated mtime blindly as "the last time inputs were parsed".
5. The touched/modified promises.cf is never parsed.

I suggest that cf_promises_validated's mtime not be used by cf-execd, or at least not populate PROMISETIME with its mtime.
Pull request filed as https://github.com/cfengine/core/pull/259